The invention relates to a switching device for powering a lamp, including
input terminals for connection to a supply voltage source,
an inverter for generating a high-frequency lamp current which includes
switching means which are coupled to the input terminals,
an external drive circuit which is coupled to the switching means in order to generate a control signal of frequency f for controlling the switching means so as to be alternately conductive and non-conductive,
a dimming circuit which is coupled to the control circuit in order to adjust the frequency f, and
a load circuit which is coupled to the switching means and includes a series connection of an inductive element and lamp connection terminals which are connected by a circuit which includes a first capacitive element.
The invention also relates to a compact lamp.
A switching device of the kind set forth is known. A user of the known switching device can adjust the light flux of a lamp powered by the switching device by adjusting the frequency of the control signal by means of the dimming circuit. The lamp can thus be dimmed by means of comparatively simple means. When the capacitance of the capacitive element is chosen to be comparatively low, it is ensured that the relationship between the frequency of the control signal and the lamp power is unambiguous throughout the power range of the lamp. However, the range in which the frequency of the control signal should be adjustable then becomes comparatively large. This gives rise to problems in practice, because the integrated circuit that is often included in the control circuit usually is not capable of generating a control signal having a comparatively high frequency. Increasing the capacitance of the capacitive element reduces the range over which the frequency must be adjustable. It is a drawback of a higher capacitance of the capacitive element, however, that the relation between the frequency of the control signal and the lamp power usually is not unambiguous throughout the power range of the lamp. In practice the dimming range is thus limited and instabilities are liable to occur in the lamp at comparatively low values of the lamp power.
It is an object of the invention to provide a switching device such that a lamp powered by means of the switching device can be dimmed over a large range by adjustment of the frequency of the control signal over a comparatively small range, the operation of the lamp nevertheless being stable throughout the dimming range.
To this end, a switching device of the kind set forth in accordance with the invention is characterized in that the inductive element is proportioned such that
L20/L maxxe2x89xa60.7,
wherein Lmax is the instantaneous value of the inductance of the inductive element when the amplitude of the current in the inductive element is maximum and the lamp power has its maximum value, and L20 is the instantaneous value of the inductance of the inductive element when the amplitude of the current in the inductive element is maximum and the lamp power amounts to 20% of its maximum value.
When the frequency of the control signal in a switching device in accordance with the invention is increased by means of the dimming circuit, the frequency of the current in the load circuit also increases. As a result, the operating voltage of the lamp and the amplitude of the current through the inductive element also increase. Due to the increase of the amplitude of the current through the inductive element, the effective value of the inductance of the inductive element decreases because the instantaneous value of the inductance is comparatively low during a part of each high-frequency period of the current through the inductive element and a considerable degree of saturation of said inductive element occurs. The decrease of the effective value of the inductance in response to an increase of the frequency of the control signal ensures that an unambiguous relation exists between the frequency of the control signal and the lamp power over a comparatively large range of the lamp power. In other words, the lamp can be dimmed over a large range and its operation is stable throughout the dimming range.
Favorable results have been found notably for embodiments of a switching device in accordance with the invention wherein L20/Lmaxxe2x89xa60,5.
Favorable results have also been found for embodiments of a switching device in accordance with the invention in which the switching means include a series connection of two switching elements so that the inverter constitutes a so-called bridge circuit.
It has also been found that the inductive element in a switching device in accordance with the invention may have a very simple construction. Attractive results have notably been achieved by means of embodiments of a switching device in accordance with the invention in which the inductive element includes an I-core provided with a winding of copper wire.
A switching device in accordance with the invention may have a comparatively simple and hence compact construction so that it is perfectly suitable for use in the electronic ballast of a compact lamp which includes
a light-transmitting discharge vessel which is provided with a filling containing a noble gas and with two electrodes,
a lamp housing which is connected to the discharge vessel,
a lamp cap which is provided with electrical contacts and is connected to the lamp housing, and
an electronic ballast which is coupled between the electrodes and the contacts for generating a lamp current from a supply voltage.
An embodiment of a switching device in accordance with the invention and an embodiment of a compact lamp in accordance with the invention will be described in detail hereinafter with reference to a drawing. Therein: